Certain petroleum refining processes, such as catalytic cracking, catalytic reforming, isomerization, etc. are carried out at elevated temperatures in the presence of a catalyst. In some of these processes coking of the catalyst occurs, i.e. coke is deposited onto the catalyst, with the result that over a period of time the catalyst gradually loses its activity. To restore the activity of the catalyst, the catalyst must be periodically regenerated, which is usually accomplished by combusting the coke at elevated temperatures in the presence of an oxygen-containing gas, such as air or oxygen-enriched air.
The catalytic process may be carried out by any one of various procedures; e.g. it may be a fixed bed process, in which case the catalytic reaction and catalyst regeneration are conducted in a single vessel, or it may be one of the moving catalyst processes, such as a transport bed process or a fluidized bed process, in which case the catalytic reaction is carried out in one vessel and catalyst regeneration is carried out in another vessel. A major advantage that moving catalyst processes have over fixed bed processes is that in moving bed processes, the reaction can be carried out continuously, whereas in fixed bed processes, the catalytic reaction must be terminated periodically to regenerate the catalyst.
In moving catalyst systems, the hydrocarbon feed and hot freshly regenerated catalyst, and perhaps steam, are continuously introduced into the reactor. The hot catalyst causes the hydrocarbon feed to react, thereby producing an array of valuable hydrocarbon products which may be of lower molecular weight than the hydrocarbon feed. During the course of the reaction the catalyst becomes fouled with coke deposits and loses its catalytic activity. The hydrocarbon products and fouled catalyst are separated and each leaves the reactor; the hydrocarbon products being sent to downstream hydrocarbon separation units to recover the various products, and the fouled catalyst being transported to a catalyst regenerator for removal of coke from the catalyst.
The effectiveness of the regenerator in burning coke off the catalyst directly determines the quality of performance of the hydrocarbon reaction (e.g. cracking) step. The regeneration step provides reactivated catalyst and heat for the endothermic hydrocarbon cracking step. The catalyst is heated during the regeneration step and the hot catalyst is transported to the reactor, where it contacts the hydrocarbon feed and causes the reactions to occur.
The amount of oxygen-containing gas (e.g. air) present in the regenerator determines the amount of coke that can be burned off the catalyst. The kinetics and efficiency of the combustion process also determines the steady-state concentrations of coke returned to the reactor on the reactivated catalyst, and the amount of coke on the spent catalyst entering the regenerator. In general, the more efficiently the catalyst is reactivated, the better its hydrocarbon reaction activity and selectivity will be, and the greater its ability to process heavier, poorer quality feedstock will be.
The rate of coke combustion is usually controlled by regulating the amount of oxygen entering the coke combustion zone during catalyst regeneration. Traditionally, catalyst regeneration has been carried out using air as the oxygen-containing gas. The nitrogen in air serves to remove heat from the reaction zone, thereby moderating the combustion. If it is desired to increase the rate of combustion, the flow of air through the regeneration zone is increased. This will have the sometimes undesirable effect of increasing the velocity of gas flowing through the combustion zone, which can cause excessive attrition and loss of the catalyst and excessive wear on equipment. To avoid these effects, some recent improvements have centered around the use of other oxygen-inert gas mixtures, such as oxygen-carbon dioxide mixtures for catalyst regeneration. Carbon dioxide has a greater heat capacity than nitrogen; accordingly the same amount of heat transfer can be effected with a lower volume of carbon dioxide than would be required using nitrogen, which means that the feed gas can be richer in oxygen. In the case of continuous regeneration processes, such as fluidized catalytic cracking, this provides an additional advantage in that additional hydrocarbon can be processed in a cracking reactor of given size. The use of oxygen-carbon dioxide mixtures in FCC units is discussed in U.S. Pat. Nos. 4,304,659 and 4,388,218. U.S. Pat. No. 4,354,925 discloses the use of mixtures of oxygen and carbon dioxide to regenerate catalytic reformer noble metal catalyst.
One of the difficulties associated with the use of oxygen-carbon dioxide mixtures is providing sources of oxygen and carbon dioxide. Oxygen can be easily generated by an on-site oxygen generator. The viability of an oxygen carbon dioxide-based regeneration process is determined by the ability to obtain carbon dioxide economically. Carbon dioxide can also be provided by recycling carbon dioxide produced during the combustion of the coke deposits, as taught in U.S. Pat. No. 4,542,114. This patent states that in some cases diluent carbon dioxide can be imported into the system.
The above-described prior art references discuss the operation of decoking processes using mixtures of pure oxygen and carbon dioxide, but none of the references discuss the most important aspect, i.e. how the operating mixture of oxygen and carbon dioxide is initially attained. The present invention provides an efficient and economical method of starting up an oxygen and carbon dioxide-based catalyst decoking process.